There is a technology with which, in communication between a base station and a terminal, the base station sends a signal by forming a beam (beam forming) only in the direction of the communication destination terminal with the use of a plurality of antennas. This technology enables the base station to limit the direction in which a wireless signal travels. Interference with another terminal located in another place can consequently be prevented or reduced.
In addition, the transmission of a wireless signal in a specific direction makes electric power for transmission concentrated compared to when a signal is transmitted to the entire service area, and the concentration can be utilized to lengthen the distance covered by the signal.
This beam forming is applicable also when a base station receives a wireless signal from a terminal. Specifically, by giving wireless signal reception directivity to a reception antenna of a base station, the influence of interference waves arriving from other directions than the direction of the reception directivity can be prevented or reduced.
However, as described above, a single beam, which has been formed in a specific direction (beam direction) through application of beam forming, is transmitted in only the beam direction and received from only the beam direction, and thus it is necessary to direct the beam direction in a plurality of directions in order to cover the entire service area of one base station. FIG. 11 is an explanatory diagram about a base station whose service area is established with the use of a plurality of beam directions. Switching from one beam direction to a plurality of beam directions, using a plurality of beams directed in different directions, or using a combination thereof is conceivable as a method of directing the beam in the plurality of directions.
In initial connection to start communication, a terminal first searches for a base station with which a signal quality high enough to hold communication is obtained. The terminal also conducts a search for nearby base stations, excluding a base station with which the terminal is currently holding communication, when what is called handover takes place in which the terminal switches from the current communication partner base station to another base station.
FIG. 12 is an explanatory diagram for illustrating a sequence of general handover in a wireless communication system in which beam forming is not used. A terminal detects quality deterioration in a signal communicated to and from a base station (communication partner base station) with which the terminal is currently holding communication. The terminal then receives notification signals transmitted from nearby base stations and measures quality of the notification signals, and sends a report on the result of the measurement to the current communication partner base station, along with a handover request.
When receiving the report, the current communication partner base station determines one of nearby base stations with which a signal quality high enough to hold communication is obtained, and requests the determined nearby base station to accept handover. The requested nearby base station sends a response informing of its acceptance to the current communication partner base station when the requested nearby base station is available for handover.
The current communication partner base station receives the response informing of the availability for handover, and transmits a handover execution command to the terminal along with information (a base station ID and the like) about the nearby base station that is the handover destination.
The terminal receives the handover execution command and transmits a random access (hereinafter referred to as “RACH signal”) as a communication request to the handover destination nearby base station. The nearby base station detects the RACH signal, and returns a random access response to the terminal to start communication.
Information about timing at which a base station can receive a RACH signal is normally contained in the base station's notification signal. The terminal can accordingly find out the RACH signal reception timing of the base station from the notification signal, and transmits a RACH signal according to the reception timing.
One of or all of the following information and signals are referred to herein as “notification signal”.                A synchronization signal required for a terminal to synchronize with a base station in frequency and time, and containing the identifier of the base station (a base station ID)        A reference signal for measuring the quality of a signal from a base station (also called a pilot signal)        Minimum information required for a terminal to establish communication to a base station, for example, information about a frequency used by the base station to receive a RACH signal and RACH signal reception timing of the base station        
A method of establishing initial connection in a system using the beam forming technology is disclosed in the related art (see Patent Literature 1, for example). In Patent Literature 1, one base station transmits a signal (search signal) for a terminal to search for the base station by sequentially changing the beam direction. The terminal receives the search signal to detect the base station, and transmits a RACH signal to the detected base station.
At this time, the base station changes a beam direction in which the search signal is to be transmitted at certain timings, and changes a beam direction in which the RACH signal is to be received as well. Accordingly, a timing at which the transmission beam of the search signal is directed in one beam direction and a timing at which the reception beam of the RACH signal is directed in the same direction have a correspondence relationship, and the search signal contains information about the timing of receiving a corresponding RACH signal.
On the basis of the descriptions of paragraphs [0048] to [0052] of Patent Literature 1, the beam direction used for transmitting a search signal and the beam direction used for receiving a RACH signal are considered to be the same direction.
It is also disclosed in Patent Literature 1 that, when a base station can form a plurality of beams simultaneously, RACH signals are received at the same reception timing by forming beams in a plurality of directions. With regard to the combination of a plurality of beams, it is only mentioned in Patent Literature 1 that a combination high in orthogonality is selected. It is also described in Patent Literature 1 that the same concept is applied to handover.